This invention relates to a process of producing single crystals of gallium phosphide from the melt containing indium as a solvent.
Upon growing single crystals of gallium phosphide (GaP) from the liquid phase, there has been generally prepared a melt including gallium(Ga), as the so-called solvent and having an amount of gallium phosphide melted thereinto to its saturated solubility at that temperature from which the growth is initiated. Then, the melt including a seed crystal is slowly cooled from a temperature of 1,000.degree.C or more to room temperature to epitaxially grow a single crystal of gallium phosphide on the seed crystal. Alternatively, the melt without a seed crystal may be slowly cooled to freely grow a dendritic crystal or crystals in the solvent. In either case, the practically usable temperature of the gallium melt is within a range of from 900.degree.C to 1,100.degree.C within which the gallium phosphide has a solubility of from 1 to 6% to the solvent, gallium. Thus, in order to grow a predetermined amount of a single crystal of gallium phosphide it is required to use the element gallium in a very large amount as compared with the required amount of the single crystal. About 5 to 50 grams of the element gallium is normally required for performing one epitaxial growth operation.
If the solvent can be formed of the element indium which is more inexpensive than the gallium, then single crystals of gallium phosphide can be formed with a decreased cost. The use of indium as a solvent for the growth of single crystals of gallium phosphide is described in U.S. Pat. No. 3,278,342 issued on Oct. 11, 1966 to H.F. John et al. The cited patent teaches the solvent, indium having melted thereinto from 5 to 15 molar percent of gallium phosphide. However, as will readily be understood, the melting of gallium phosphide into the element indium leads to the growth of mixed crystals expressed by In.sub.x Ga.sub.1-x P where x varies between 0 and 1 but not of single crystals of gallium phosphide. The mixed crystal expressed by In.sub.x Ga.sub.1-x P is composed of InP crystals and GaP crystals in a molar ratio of x/(1-x). On the other hand, lattice constants of the InP and GaP crystals are of 5.8687 and 5.4505A respectively so that a difference therebetween is great. Thus, such a mixed crystal is normally bad in crystallographic properties. Particularly, the crystallographic properties are much deteriorated for the x ranging from 0.2 to 0.8. Also the two are different in the forbidden band from each other and therefore they can not be equally utilized. For example, it has been experimentally found that with from 5 to 10 molar percent of gallium phosphide melted into the solvent, indium, the resulting mixed crystal has a value of the x approximately equal to 0.2 and becomes normally unhomogeneous in the composition within a single crystal. This has resulted in the deterioration of the crystallographic properties while differentiating the wavelengths of light emitted therefrom from those exhibited by the gallium phosphide crystal. Thus it is difficult to use mixed crystals In.sub.x Ga.sub.1-x P as a material of semiconductor light-emitting diodes, assuming that it is equivalent to the single crystal of gallium phosphide.
Accordingly it is an object of the present invention to provide an improved process of producing single crystals of gallium phosphide from a melt at a low cost by minimizing an amount of a raw material not contributing directly to the formation of the single crystals.